Pharmacokinetics, Safety, and Tolerability of Tenapanor in Healthy Chinese and Caucasian Volunteers: A Randomized, Open-Label, Single-Center, Placebo-Controlled Phase 1 Study

Background Tenapanor is a locally acting selective sodium-hydrogen exchanger 3 inhibitor with the potential to treat sodium/phosphorus and fluid overload in various cardiac-renal diseases, which has been approved for constipation-predominant irritable bowel syndrome in the US. The pharmacokinetics (PK) of tenapanor and its metabolite tenapanor-M1 (AZ13792925), as well as the safety and tolerability of tenapanor, were investigated in healthy Chinese and Caucasian subjects. Methods This randomized, open-label, single-center, placebo-controlled phase 1 study (https://www.chinadrugtrials.org.cn; CTR20201783) enrolled Chinese and Caucasian healthy volunteers into 4 parallel cohorts (3 cohorts for Chinese subjects, 1 cohort for Caucasian subjects). In each cohort, 15 subjects were expected to be included and received oral tenapanor (10 or 30 mg as single dose, or 50 mg as a single dose followed by a twice-daily repeated dose from Day 5 to 11, with a single dose in the morning on Day 11) or placebo in a 4 : 1 ratio. Results 59 healthy volunteers received tenapanor 10 mg (n = 12 Chinese), 30 mg (n = 12 Chinese), or 50 mg (n = 12 (Chinese), n = 11 (Caucasian)) or placebo (n = 12, 3 per cohort). After single and twice-daily repeated doses, tenapanor plasma concentrations were all below the limit of quantitation; tenapanor-M1 appeared slowly in plasma. In single-ascending dose evaluation (10 to 50 mg) of Chinese subjects, the mean Cmax, AUC0-t, and AUC0-∞ of tenapanor-M1 increased with increasing dose level, and AUC0-t increased approximately dose proportionally. The Cmax accumulation ratio was 1.55 to 6.92 after 50 mg repeated dose in Chinese and Caucasian subjects. Exposure to tenapanor-M1 was generally similar between the Chinese and Caucasian subjects. Tenapanor was generally well-tolerated and the safety profile was similar between the Chinese and Caucasian participants receiving tenapanor 50 mg, as measured by vital signs, physical and laboratory examination, 12-lead ECG, and adverse events. No serious adverse event or adverse event leading to withdrawal occurred. Conclusion Tenapanor was well-tolerated, with similar PK and safety profiles between Chinese and Caucasian subjects. This trial is registered with CTR20201783.


Background
Globally, the prevalence of all-stage chronic kidney disease (CKD) was 9.1% in 2017.A substantial increase was noted in the incidence of end-stage renal disease (ESRD) in individuals receiving renal replacement therapy [1].Te ability to excrete phosphorus gradually diminishes with the progressive impairment of renal function.Consequently, hyperphosphatemia is encountered commonly in ESRD patients.
Hyperphosphatemia is known to be associated with vascular calcifcation, cardiovascular events, and mortality among patients with CKD, especially in those receiving maintenance hemodialysis [2][3][4].Terefore, practice guidelines recommend that serum phosphate levels should be maintained within the normal range, i.e., 3.5 to 5.5 mg/dL [5,6].However, due to the complexity of phosphate metabolism regulation and the practical challenges of currently available therapeutic modalities, the management of hyperphosphatemia remains unsuccessful in many patients [7,8].Te development of oral phosphorus-lowering drugs that possess novel mechanisms of action which allow for easier delivery and improved beneft-risk profles is needed urgently.
Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder that is associated with a signifcant healthcare burden; for some patients, IBS symptoms can be disabling [9].Te prevalence of IBS ranges from 2% to 6% in most countries [10].IBS patients are categorized as those having a tendency for predominant diarrhea (IBS-D) or those with predominant constipation (IBS-C).Features of IBS pathogenesis include changes in fecal microbial fora and gastrointestinal motility, post-infectious reactivity, brain-gut interactions, overgrowth of bacteria, sensitivity to food, malabsorption of carbohydrate, and intestinal infammation [11].Increased understanding of the molecular and pathophysiological pathways of IBS has led to the development of new pharmacological agents and the identifcation of potential novel therapeutic targets [9].
Te sodium-hydrogen exchanger 3 (NHE3) is important for intestinal sodium transport and subsequent fuid homeostasis.NHE3 inhibition in the gut by tenapanor, a minimally absorbed selective NHE3 inhibitor, decreases gastrointestinal sodium absorption which results in an elevation in stool fuid content [12] and a reduction in the paracellular absorption of luminal phosphate [13,14].Tus, tenapanor may be useful in the treatment of sodium/ phosphorus and fuid overload in various cardiac-renal diseases as well as in constipation-related disorders.
Following on from positive results obtained in the phase 3 T3MPO trial, tenapanor was approved by the US Food and Drug Administration for the treatment of adults with IBS-C [15].In phase 1 to phase 3 trials, tenapanor, both as monotherapy and in combination with an established phosphate binder regimen, signifcantly reduced serum phosphorus concentrations in hyperphosphatemic patients who were receiving maintenance dialysis [16][17][18][19].Tenapanor monotherapy reduced concentrations of serum phosphorus by a similar magnitude to that of an established phosphate binder [17,19].
Potential diferences in pharmacokinetics across ethnic groups may afect drug efcacy and safety and the feasibility of generalizing the fndings of pivotal studies to particular populations.To date, the pharmacokinetics (PK) and safety profle of tenapanor in the Chinese population have not been established.Here, we report the PK of tenapanor and its major inactive metabolite, tenapanor-M1 (AZ13792925), as well as the tolerability and safety of tenapanor, in healthy Chinese and Caucasian individuals in a phase 1 study.

Study Design.
Tis was an open-label, single-center, randomized, placebo-controlled phase 1 study (Identifer: CTR20201783).In this study, 60 Chinese and Caucasian healthy volunteers were expected to be recruited, who were enrolled into 4 parallel cohorts (cohort 1 to 3 for Chinese subjects, cohort 4 for Caucasian subjects, n � 15 in each cohort).For cohorts 1 and 2, participants were randomized in a 4 : 1 ratio to receive one single dose of tenapanor (10 or 30 mg) or placebo.For cohorts 3 and 4, participants received tenapanor 50 mg (or the corresponding placebo, in a 4 : 1 ratio) as a single dose on Day 1, followed by a repeated dose of the same tablets (twice daily, from Day 5 to 11, with only a single dose given in the morning on Day 11).Te study was conducted according to the principles contained in the Declaration of Helsinki.All participants provided the written informed consent.

Participants.
Chinese or Caucasian volunteers were eligible if they were (1) a resident of China; (2) able to understand and adhere to the protocol, with written informed consent; (3) aged between 18 and 45 years; (4) not less than 45 kg (female) or 50 kg (male) in body weight with a body mass index (BMI) of 18.5 to 28.0 kg/m 2 ; (5) healthy, as measured by vital signs, physical and laboratory examination, 12-lead electrocardiography (ECG), chest X-ray, and infectious disease screening in the 2-week period before inclusion.A negative pregnancy test was required for female subjects.
Key exclusion criteria included the following: a history or presence of clinically signifcant hepatic, gastrointestinal, or renal disease, including gastrointestinal surgery (other than appendectomy) or any other condition known to interfere with the study drugs; loose stools (Bristol Stool Form Scale score [20] of 6 or 7) for ≥2 days in the 7 days before inclusion; use of medications or supplements in the 14-day period before assignment to treatment; and abuse of tobacco, alcohol, or medications.

Study Drug Administration.
Subjects were screened within 14 days prior to the baseline period.After enrollment, subjects were admitted to the Phase I Clinical Trial Center at the First Afliated Hospital, Sun Yat-sen University, Guangzhou, Guangdong, China, and were given a randomization number.
In cohort 1 (10 mg) and cohort 2 (30 mg), Chinese subjects received either tenapanor (one or three tablets of tenapanor 10 mg, respectively) or placebo on Day 1.In cohort 3 (50 mg-Chinese) and cohort 4 (50 mg-Caucasian), subjects received one tablet of tenapanor 50 mg or placebo on Day 1, followed by a repeated dose with the same tablet twice daily for 7 days (from Day 5 to 11, with only a single dose given in the morning on Day 11).
Tenapanor tablets or placebo were delivered with 240 mL of water in the morning of the next day (Day 1) after at least 10 hours of fasting.For all single-dose evaluations, tablets were administrated 5 to 10 minutes before breakfast on Day 1.For the repeated-dose evaluation, tablets were administrated 5 to 10 minutes before breakfast and supper from Day 5 to Day 11 (only in the morning on Day 11).Standard meals were provided by the clinical center.Subjects took the assigned food at the appointed time during the hospital stay and did not take any other diet.Blood samples for PK analysis were collected within 1 hour at pre-dose and at 1, 2, 4, 6, 7, 8, 9, 10, 12, 24, 48, and 72 hours post-dose, on Day 1 for single-dose analysis, or on Day 11 for repeated-dose analysis.Plasma concentrations of tenapanor and tenapanor-M1 were quantifed by liquid chromatography-tandem mass spectrometry (LC-MS/MS; Sciex Triple Quad 6500+, Framingham, MA, USA) using a HPLC column (Poroshell HPH-C18, 4 µm, 4.6 × 50 mm, Agilent, Santa Clara, CA, USA), and mobile phase A (5 mM ammonium formate and 0.1% formic acid in purifed water) and B (0.1% formic acid in acetonitrile), maintained at 40 °C.Te gradient elution was delivered as follows (A : B): 0.01-3.00min: 70 : 30-40 : 60; 3.00-3.05min: 40 : 60-5 : 95; 3.05-4.50min: 5 : 95-5 : 95; 4.50-4.55min: 5 : 95-70 : 30.Te fow rate was 0.6 mL/min for 0-3.05 min, 1.2 mL/min for 3.10-4.50min, and 0.6 mL/min for 4.55-6.00min.Isotopelabeled tenapanor-d8 and [13C]6-M1 were used as internal standards.Te lower and upper limits of quantifcation were 0.5 and 300 ng/mL for tenapanor and M1, respectively.AEs were recorded according to the Medical Dictionary for Regulatory Activities (MedDRA) version 24.1.Vital signs including blood pressure, heart rate, and body temperature were collected at screening, during the study and before subject discharge.Physical and laboratory examinations, as well as ECGs, were conducted at screening and before subject discharge.

Statistical Analysis. PK analysis was conducted
according to the noncompartmental method using Phoenix software (version 8.3, Pharsight Corporation, Mountain View, CA, USA).SAS (version 9.4, SAS Institute Inc., Cary, NC, USA) was used for statistical analysis.
For each group, continuous data were summarized using the number of subjects, mean, standard deviation (SD), median, and minimum and maximum values.Categorical data were summarized using frequencies and percentages.Geometric mean and coefcient of variation (CV) were used for applicable plasma concentrations and PK parameters.Te linear PK characteristics of PK parameters were evaluated by a power function model based on confdence interval.Dose proportionality for C max , AUC 0-t , and AUC 0-∞ was evaluated by power function model, according to Hummel standard (judgment interval: [1 + ln(θ L )/ln(r), 1 + ln(θ H )/ln(r)], θ L � 0.5, θ H � 2.0 and r � maximum dose/ minimum dose) based on comparing the 95% confdence interval for the ratio of predicted mean values from the extremes of the dose range to pre-defned equivalence criterion [21][22][23].
Due to the large variation in PK parameters of tenapanor and the small number of subjects in each group, statistical analysis of PK parameters was mainly descriptive with no formal statistical assumption.A comparison of ethnic differences in the PK profle was conducted in Chinese and Caucasian subjects in the 50 mg dose group, based on boxand-whisker plots of the main PK parameters, the geometric mean ratio (GMR) of parameters between diferent ethnic groups, and their 90% confdence intervals (PK parameters were considered similar in the two ethnic groups if the 90% confdence intervals for the GMRs of PK parameters crossed 1.00 as well as the point estimate of the GMRs were between 0.80 and 1.25), combined with clinical relevance.

Study Participants.
Of 310 healthy subjects screened, 59 subjects were enrolled in the study, including 45 Chinese and 14 Caucasian subjects.Te mean (±SD) age of subjects was 27.4 ± 5.04 years, 39 (66.1%) were male.In general, the demographic and baseline characteristics of subjects were well balanced across all treatment groups (Table 1).

Single-Ascending Dose (SAD).
Te tenapanor plasma concentration was below the lower limit of quantitation (LLOQ, 0.5 ng/mL) in all Chinese and Caucasian subjects receiving a single dose of tenapanor 10, 30, or 50 mg.Te major inactive metabolite, tenapanor-M1, appeared slowly in plasma following SAD administration of tenapanor 10 to 50 mg, with a median T max between 7 and 8.458 hours (Figure 1(a); Table 2).Te plasma concentration of tenapanor-M1 increased with increasing dose levels.In SAD evaluation (10 mg to 50 mg in Chinese subjects), the slope of AUC 0-t was 0.98, close to 1, and the 95% confdence interval was 0.74 to 1.22.It falls within the target interval of 0.57 to 1.43, which indicated that AUC 0-t of tenapanor-M1 from 10 mg to 50 mg was approximately linear, according to Hummel standard [21][22][23].While AUC 0-∞ and C max increased slightly less than dose proportionally (Table 3 and Supplementary Figure 1).After the administration of tenapanor, tenapanor-M1 had a long half-life, with a geometric mean t 1/2 between 16.37 and 20.61 hours.After a single dose of tenapanor 10 mg, 30 mg, or 50 mg in Chinese subjects, the geometric mean C max of tenapanor-M1 was 1.7294, 3.4924, and 5.5194 ng/mL, respectively, and the geometric mean AUC 0-t was 34.1165, 90.9420, and 169.6988 hour * ng/mL, respectively.Te geometric mean C max and AUC 0-t of tenapanor-M1 were 5.2455 ng/mL and 129.4200 hour * ng/mL, respectively, after a single dose of tenapanor 50 mg in Caucasian subjects.2), respectively.Te geometric mean accumulation ratio of C max was 3.47 (range, 1.55 to 6.78) in Chinese subjects and 3.36 (range, 1.98 to 6.92) in Caucasian subjects, after repeated doses of tenapanor 50 mg.

Ethnic Diferences.
Exposure to tenapanor-M1, as assessed by mean C max and mean AUC, was generally similar in Chinese and Caucasian subjects after administration of tenapanor 50 mg as single or repeated doses (Figure 1).Te geometric mean ratio (GMR) of C max in Chinese subjects relative to Caucasian subjects was 1.05 (90% CI, 0.78 to 1.41) after tenapanor 50 mg as a single dose and 1.09 (90% CI, 0.88 to 1.34) after repeat-dose tenapanor 50 mg.Te GMR of AUC 0-t was 1.31 (90% CI, 0.92 to 1.87) and the GMR of AUC 0-∞ was 1.13 (90% CI, 0.84 to 1.53) after single-dose tenapanor 50 mg.Te GMR of AUC 0-τ was 1.10 (90% CI, 0.89 to 1.37) after repeat-dose tenapanor 50 mg.PK parameters in Chinese and Caucasian subjects after single and repeated doses of tenapanor 50 mg are shown in Supplementary Table 1.

Safety and Tolerability.
No signifcant changes or trends in laboratory tests, vital signs, ECG, or physical examination fndings were observed after single and/or repeated doses of tenopanor in each dose cohort.
AEs that occurred in each treatment group are summarized in Table 4. Te incidence of treatment-emergent AEs (TEAEs) of any grade was 41.7%, 58.3%, and 100% in subjects receiving tenapanor 10, 30, or 50 mg (single or repeated doses).Six of 12 (50%) subjects in the placebo group experienced at least 1 TEAE.
TEAEs reported most commonly in subjects receiving tenapanor at any dose level were gastrointestinal disorders, including diarrhea, abdominal distention, upper abdominal pain, and dyspepsia.Diarrhea was considered a tenapanor treatment-related AE (TRAE) and occurred more frequently in subjects receiving tenapanor 50 mg compared to those who received placebo.Te incidence of diarrhea and other International Journal of Clinical Practice

Discussion
An important feature of the gastrointestinal tract is to maintain a stable internal environment through balancing the secretion and absorption of fuids and sodium.While the role of NHE3 in intestinal sodium absorption is critical, the mechanism of action of tenapanor to target NHE3 in the gastrointestinal tract results in an increase in stool fuid content as well as a reduction in the absorption of luminal phosphate [12].Tis important feature of tenapanor forms the basis for treating various diseases, including constipation-related disorders and hyperphosphatemia.
In our study, we characterized the PK of tenapanor and its major inactivate metabolite in Chinese and Caucasian healthy volunteers.Te tenapanor plasma concentration was below the LLOQ in all blood samples collected from all subjects up to a dose of 50 mg, aligning with data reported from other studies in healthy volunteers [12,16,24].In contrast, the major inactive metabolite, tenapanor-M1, was detectable and appeared slowly in plasma.In SAD evaluation, the AUC 0-t of tenapanor-M1 from 10 mg to 50 mg was approximately linear [21][22][23], while AUC 0-∞ and C max increased in a manner that was slightly less than dose proportional.Te PK profle of tenapanor-M1 was numerically similar with the results observed in a phase 1 Japanese study, in which the median T max was about 4 to 8 hours after single or repeated tenapanor doses of 15 to 90 mg, with an accumulation ratio of C max of 5.00 to 8.18 after repeated doses (at multiple dose levels), and the average C max and AUC of tenapanor-M1 also increased in an approximately dose proportional way (unpublished data; Supplementary Figure 2).Moreover, the C max in healthy Japanese subjects was close to that in healthy Chinese subjects, with geometric (CV%) C max values of 2.47 (53.3) and 3.49 (38.18) ng/mL, respectively, after tenapanor 30 mg as a single dose.
Overall, the PK profles of tenapanor in Chinese and Caucasian subjects were similar.Te 90% CI of the C max and AUC GMRs all crossed 1.00.Most of the GMR point estimates of the PK exposure parameters (i.e., C max and AUC 0-∞ after single oral dose and C ss , C max,ss , C min,ss , C av,ss , and AUC 0-τ after repeated oral dose) for Chinese or Caucasian subjects were within the range of 0.80 to 1.25.Te only exception was the point estimate of the GMR for AUC 0-t of tenapanor-M1 after single oral administration, which was 1.31; this may be partially explained by the small sample size, the uneven number of cases, and the low tenapanor-M1 plasma concentration, which was already lower than the LLOQ in some subjects at 48 or 72 hours.Moreover, minor diferences in tenapanor-M1 exposure levels between Chinese and Caucasian subjects did not cause clinically meaningful diferences in safety between the two groups.Our observations were in line with data reported from other studies, such as in a phase 1 study showing that the safety profle and pharmacodynamic efects of tenapanor were  International Journal of Clinical Practice similar in Caucasian and Japanese healthy volunteers [16].
In addition, exposure to tenapanor-M1 (as assessed by mean C max and AUC 0-8 ) for Japanese and Caucasian subjects was generally similar at the 90 mg twice daily dose (unpublished data).
In our study, tenapanor showed a good safety profle in healthy volunteers.At 10, 30, and 50 mg single-dose and 50 mg repeated-dose levels, tenapanor was generally welltolerated in Chinese and Caucasian healthy volunteers.Te most commonly reported AEs were gastrointestinal disorders; diarrhea was the most common event and this is consistent with the fndings from other studies in healthy volunteers [12,16,24].According to the reported study in Japanese population [16], two events of treatment-related diarrhea (60 and 90 mg twice daily, one per group, 8.3%) occurred in healthy Japanese subjects.While 12/12 (100%) events of all-cause diarrhea occurred in 50 mg twice daily group in Chinses subjects in our study.Tis discrepancy is probably due to the diferent criteria for evaluating diarrhea as well as the fact that the Japanese study also examined the pharmacodynamics of tenapanor, where the subjects were more restricted to standard meals, which may afect the incidence of diarrhea.As a result of the pharmacological efect of tenapanor, leading to increased gastrointestinal tract water retention, diarrhea is an expected AE.All reported diarrhea events in this study were mild in severity and resolved with symptomatic intervention (such as montmorillonite), which is also consistent with the observation of rare discontinuations or dose adjustment of tenapanor due to diarrhea in other studies [12,16,17,25].
Te safety of tenapanor in patients with renal impairment has been evaluated in three phase 3 studies including patients with ESRD who were receiving maintenance hemodialysis [17,18].In the TEN-02-201 trial, TRAEs were reported in 46.5% of patients receiving tenapanor 30 mg twice daily in the randomized treatment period.SAEs occurred in 7.0% of patients, and 15.5% of patients experienced AEs leading to discontinuation [17].In the AMPLIFY trial, treatment-related diarrhea occurred in 40.2% of patients receiving tenapanor + phosphate binder, compared with 6.7% of patients in the placebo + phosphate binder group.AEs leading to study drug discontinuation occurred in 4.3% and 2.5% of patients in the two groups, respectively [18].Te safety of tenapanor in the Chinese population with renal impairment was similar with the studies of TEN-02-201 and AMPLIFY.Diarrhoea was the predominant TRAE (28.6%) and most was mild in severity [26].
CKD has become an important public health burden in China [27].Given that hyperphosphatemia is an important complication in CKD patients that can result in unfavorable clinical consequences, there is currently an unmet need for efective and safe phosphorus-lowering drugs which act through novel mechanisms, especially for the vast number of Chinese patients.As demonstrated in pivotal phase 2 and 3 clinical trials, tenapanor, both as monotherapy and in combination with an established phosphate binder regimen, decreased concentrations of serum phosphorus signifcantly in patients who were receiving maintenance dialysis [17-19, 25, 26].
Tere were some limitations in our study.As a molecule that acts locally in the intestine, tenapanor absorption into the circulation is known to be minimal.Tenapanor-M1, the only major inactive metabolite of the prototype drug tenapanor, was quantifable in plasma at a low concentration.Tus, the PK comparison data should be interpreted with caution.Nevertheless, our results and conclusions are in accordance with previous observations from a study which compared Japanese and Caucasian subjects [16].Second, the PK and safety profle of a repeated dose of tenapanor were not evaluated at lower dose levels (e.g., 30 mg).

Conclusions
Tenapanor was safe and well-tolerated in healthy Chinese volunteers.Similar PK and safety profles were observed between Chinese and Caucasian subjects.Tese results support further clinical investigations of tenapanor in the Chinese population.

Figure 1 :
Figure 1: Pharmacokinetic characteristics of tenapanor-M1 following administration of tenapanor.(a, b) Mean plasma concentration of tenapanor-M1 (semi-log plot) following single dose (a) and repeated dose (b).(c-g) C max and AUC of tenapanor-M1 following 50 mg single dose (c-e) and repeated dose (f, g) administration of tenapanor in Chinese and Caucasian subjects.Te fve horizontal lines from the bottom to the top of the box with whisker indicate minimum without outliers, Q1, median, Q3, and maximum without outliers.Te triangle or circle inside the box represents the mean value.Te triangle or circle outside the box represents the potential outliers.
/F).For repeated-dose evaluation, parameters included T max (T max ,ss ), plasma concentration (C ss ), C max (C max ,ss ), average plasma concentration (C av,ss ), trough concentration (C min ,ss ), and CL/F (CL ss /F) at steady state, as well as AUC at steady state over the dosing interval (AUC 0-τ ), t 1/2 , and degree of fuctuation (DF).Secondary endpoints included safety and tolerability, as measured by vital signs, physical and laboratory examination, 12-lead ECG, and adverse events (AEs).

Table 1 :
Demographic and baseline characteristics of Chinese and Caucasian participants.Data are shown as mean ± SD, unless otherwise specifed. Note.

Table 2 :
Main PK parameters of tenapanor-M1 following single and repeated dose administration of tenapanor in healthy subjects.
AUC area under curve, CV coefcient of variation, DF degree of fuctuation, SD standard deviation.a Median (range).b

Table 4 :
Summary of adverse events.